1. Field of the Invention
The present invention relates to a fluorescence detection platform, and more particularly, but not by way of limitation, to a fluorescence detection platform based on the integration of grating-assisted surface plasmon coupled emission.
2. Description of the Prior Art
The phenomenon known as surface plasmon-coupled emission (SPCE) involves the coupling of surface plasmon resonance (SPR) and nearby spontaneous emission. Since the observance of SPCE in 1975, an abundance of literature containing both experimental and theoretical works has been recorded. It has been shown that, for example, the fluorescence of molecules nearby a metal film can be coupled into the surface plasmon (SP) mode, re-radiated out penetrating through the metal film into a higher refractive index substrate, and then focused at a well-defined angle. This angle generally corresponds to the “minimum total reflection angle”, which is often used to generate SPR in the Kretschmann configuration (i.e., the SPCE emission angle is the same as the SPR excitation angle with the same wavelength). Since there is a one-dimensional confinement applied to the emission, the SPCE radiation pattern is a cone 10, as shown for example in FIG. 1.
Various experiments demonstrating SPCE have been conducted. An example of an experiment setup conducted by Weber is shown above in FIGS. 1 and 2. The experiment setup includes a rhodamine-methanol solution 12 excited by a p-polarized Ar laser (514.5 nm) through a high refractive index prism 14 and a 75-nm thick Ag film 16. Since the transmission spectral bandwidth of the emission through a filter 17 is broad, the range of the corresponding SP angle is also large, and the emission cone 10 has a thick width. When this emission cone 10 is projected onto a linear photograph plate 18, a wide band is shown. If a color film is used for the same experiment, it will show rainbow-like color bands. This is essentially an “SPR grating”, which emits light of different colors into different angles.
The SPR grating effect and the preferential emission of SPCE through metal thin film into the high refractive index (prism) side can be understood by the wavenumber (also known as the propagation constant or momentum) matching criteria:k0np sin θf=Re[ksp],  (1)where k0, ksp, np, and θf are the free-space wavenumber, SP wavenumber, refractive index of the prism, and SPCE emission angle, respectively. As the SP wavenumber,
      k    sp    =            k      0        ⁢                                        ɛ            m                    ⁢                      ɛ            s                                                ɛ            m                    +                      ɛ            s                              (where εm, εs are the permittivities of the metal and sample solution), changes at different emission wavelength due to the material dispersion, the emission angle shifts correspondently.
SPCE can be modeled as a dipole emitter located in the proximity of a metal structure. In particular, FIG. 3 shows a SPCE emission pattern numerically simulated by using the two-dimensional (2D) finite difference time domain (FDTD) method. Black vertical lines 20 indicate the boundaries of the metal layer. To the left side of the black vertical lines 20 is free space 22, and the right side is filled with high refractive index material 24 (e.g., glass). A dipole normal to the metal surface emits preferentially to the high refractive index side at the SPR excitation angle.
The potential applications of SPCE are significant since any spontaneous emission process, such as fluorescence, fluorescence resonance energy transfer (FRET), Raman scattering, second harmonic generation (SHG), etc., may take advantage of the SPCE phenomenon. More particularly for fluorescence-related sensing applications, the use of SPCE offers several interesting and highly desirable features. Such features include:                Highly efficient fluorescence collection        Background noise reduction        Fluorescence intensity enhancement        Photostability improvement        
Generally, due to the isotropic emission pattern of fluorescence, the common spontaneous emission collection efficiency is about 1%. However, the SPCE has an emission pattern that essentially focuses itself into a narrower spatial distribution. Although, only the vertically oriented dipole has a high probability of SPCE coupling. As such, the averaging effect (among vertical and horizontal orientation) dilutes the maximum harvest efficiency of the traditional SPCE technique to about 50-60%. This emission confinement not only improves the signal collection efficiency, but also reduces background noise due to reduced data collection volume, near-field coupling, and possible polarization discrimination.
SPCE is categorized as the so-called “forbidden light” 30 detection scheme. When ambient light 32 enters a higher-refractive-index material 34, the transmitted light is only allowed to go into a transmission angle 36 smaller than the critical angle 38. As shown in FIG. 4, no other light besides the emission originated at the proximity of the material interface can be coupled into the forbidden region 30 (i.e., the transmission angle is greater than the critical angle). Therefore, forbidden light 31 has ultra-high signal to noise ratio (SNR) due to the very dark background and only the presence of the dipole radiation close to the interface (which is the intended signal). Since a major portion of the SPCE is focused into an emission angle 40 greater than the critical angle 38, SPCE is an excellent candidate for the forbidden light detection scheme. In addition, SPCE light is p-polarized with great polarization purity, which enables further boosting of SNR via polarization filtering.
The effect on the emission of presenting a metal surface to fluorescencing molecules can be quite different depending on the newly created photonic mode density (PMD). The presence of metal can introduce a new pathway or alter the existing mechanism of the decay processes, which may either boost the radiative rate and hence fluorescence enhancement, or promote non-radiative decay and hence fluorescence quenching. Many theoretical works have been conducted to understand this behavior.
Fluorescence is a radiative decay process, which competes with several other nonradiative decay mechanisms. It is well known that the radiative decay can be engineered by the surrounding PMD. The spontaneous emission usually interacts with the PMD via the evanescent field (near field). Enderlein has conducted some theoretical studies about such effect by modeling it as the dipole-interface interaction using the semi-classical electromagnetic theory. The modification of FRET due to the changing PMD can also be modeled as the resonance dipole-dipole interaction (RDDI) using a similar approach. However, the evanescent field generated near the noble metal surface via SPR is stronger and extends farther than the one near the dielectric interface via total internal refection (TIR). Therefore, the modification of the emission due to SPR is more dramatic than its counterpart produced by refractive index contrast. This fact grants SPCE, which utilizes SPR, a stronger improvement over other techniques of coupling spontaneous emissions.
The fluorescence intensity enhancement of SPCE comes from several factors. First, the excitation may be locally amplified via the SPR enhancement. Secondly, the radiative lifetime may be reduced due to the change of the PMD around the emitter, and hence the quantum yield efficiency is increased. Finally, the non-radiative decay is reduced in the competitive process, which results in less damage onto fluorescencing molecules and improves photostability.
The probability of the SP-coupled radiative decay rate has been calculated by Weber in 1979. FIG. 5 shows that the probability SPCE can be as high as 93% when the fluorescing molecule is vertically oriented and 120-nm away from the metal surface. However, the horizontally oriented molecule has much lower SPCE coupling probability under the same condition. In an orientation-randomized scenario, the averaged SPCE is presented as a dashed line in FIG. 5.
Ford et al. also published similar calculations in 1984. FIG. 6 presents the relative decay probability of both non-radiative and radiative mechanisms. It can be seen that the non-radiative decay (dotted lines) dominates when the molecule-metal separation is under 10-nm for both vertically (⊥) and horizontally (∥) oriented molecules. As the separation increases, the SPCE-related decay (solid lines) rises up quickly for the vertically oriented molecule, but not for the horizontal one.
The SPCE coupling range can be rather long such that it actually peaks at around 120 nm, which agrees well with Weber's results. The free-space non-SPCE related radiative decay also has been presented as dashed lines in FIG. 6. At about 120-nm away from metal surface, the free-space radiative decay actually dominates for the horizontally oriented molecules. Therefore, the preferential emission characteristic is “diluted” in the orientation-randomized case. Both works discussed above have been cited by Lakowicz, who estimates the emission harvest efficiency can still be up to 60% under the averaged results.
Ultimately, these previous studies indicate that the presence of metal should be able to enhance the fluorescence as long as the separation of the fluorophores and metal surface is well controlled. Several other experimental works have successfully demonstrated this capability.
As can be seen from the above discussions, the SPCE phenomenon has significant potential for harvesting and sensing spontaneous emission processes, particularly fluorescence emissions, with extraordinary SNR. While SPCE techniques offer some advantages, there is still a need for a compact optical collection system to collect the wide-angle emission produce by SPCE. It is to such a system and method that the present invention is directed.